The normal.y = cosine(angle) method seems the best way. A couple of other
things I have found useful in tree placement are:

Store in an array where all the trees are and check when creating a new one
that you're not creating any that are too close to other trees.

Shrink the trees by an amount dependant on the slope of the ground (+- a
random factor to keep things looking natural). Also possibly instead of
banning trees from being on steep slopes, make the probability increasingly
less as the gradient of the slope gets greater. These stop forests having
such a clearly defined outline.

-Chris

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